Process and plant for the liquefaction of air and for the storage and recovery of electrical energy

ABSTRACT

A process and plant for the liquefaction of air in which a stream of pressurized air is provided at a first pressure level and is compressed by means of a compressor to a second pressure level, a first partial stream, a second partial stream and a third partial stream are formed from air of the stream of pressurized air after the compression to the second pressure level, air of the first partial stream is cooled down using cold that is produced by means of an expansion of air of the second partial stream and the third partial stream and is at least partially liquefied, and feed air that is compressed to the first pressure level and air of the second partial stream and the third partial stream that is provided at the first pressure level are used for providing the stream of pressurized air at the first pressure level.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from European Patent Application EP 14004153A filed on Dec. 9, 2014.

BACKGROUND OF THE INVENTION

The invention relates to a process and a plant for the liquefaction of air and for the storage and recovery of electrical energy according to the respective preambles of the independent patent claims.

It is known for example from DE 31 39 567 A1 and EP 1 989 400 A1 to use liquid air or liquid nitrogen, that is to say low-temperature liquefaction products, for grid control and provision of control capacity in electric power grids.

At times when electric power is inexpensive, or at times when there is a surplus of electric power, air is liquefied in an air liquefaction plant, which may also be part of an air separation plant, altogether or partially to form such an air liquefaction product. The air liquefaction product is stored in a system of tanks comprising low-temperature tanks. This operating mode takes place in a time period that is referred to here as the energy storage period.

At times of peak load, the liquefaction product is removed from the system of tanks, the pressure is increased by means of a pump and it is heated up to approximately ambient temperature or above, and is thereby transformed into a gaseous or supercritical state. A pressurized stream thereby obtained is allowed to expand to ambient pressure in an expansion machine or a number of expansion machines with intermediate heating. The mechanical power thereby released is transformed into electrical energy in one or more generators and is fed into an electric grid. This operating mode takes place in a time period that is referred to here as the energy recovery period.

There are also known compressed-air storage power plants, in which however the air is not liquefied but compressed in a compressor and stored in an underground cavern. At times when the demand for power is high, the compressed air is conducted out of the cavern into the combustion chamber of a gas turbine. At the same time, fuel, for example natural gas, is fed to the gas turbine by way of a gas line and is burned in the atmosphere formed by the compressed air. The exhaust gas that is formed is allowed to expand in the gas turbine, whereby energy is generated.

EP 1 205 721 A discloses a process and an apparatus for the production of a cryogenic liquid, in which a compressor arrangement is used, in which two compressor stages are arranged on each shaft. The shafts bear pinions which are impinged by a torque via a drive wheel.

US 2011/0132032 A1 discloses a process and an apparatus for liquefaction and storage of air for later usage. During the later usage, the liquefied air is pressurized in liquid state, warmed and expanded. Cold can be stored.

According to U.S. Pat. No. 6,666,048 B1, for increasing the formation of a certain product in a plant, e.g. in an air separation unit, an additional plant or plant component, e.g. an absorption column, is integrated in the existing plant.

WO 2014/006426 A2 relates to a liquefaction apparatus with a heat exchanger, a first phase separator, a first expansion unit, a first expansion turbine, a second expansion turbine and a cold recovery pipeline with a heat transfer fluid.

In particular on account of the large amounts of air to be liquefied, known air liquefaction plants often do not operate efficiently enough for the stated tasks, as also still to be explained below, so that there is a need for improvements. The same also applies correspondingly to air liquefaction plants for purposes other than the storage and recovery of energy.

SUMMARY OF THE INVENTION

This object is achieved by a process and a plant for the liquefaction of air and for the storage and recovery of electrical energy. The invention provides for a process for the liquefaction of air in which a stream of pressurized air is provided at a first pressure level and is compressed using a compressor to a second pressure level, a first partial stream, a second partial stream and a third partial stream are formed from air of the stream of pressurized air after the compression to the second pressure level, air of the first partial stream is cooled down using cold that is produced by means of an expansion of air of the second partial stream and the third partial stream and is at least partially liquefied, and both feed air that is compressed to the first pressure level and air of the second partial stream and the third partial stream that is provided at the first pressure level are used for providing the stream of pressurized air at the first pressure level, the air of the second partial stream being successively cooled down to a first temperature level, allowed to expand from the second pressure level to the first pressure level and heated with respect to the first partial stream, and the air of the third partial stream being successively cooled down to a second temperature level below the first temperature level, allowed to expand to a third pressure level below the first pressure level, heated with respect to the first partial stream and recompressed to the first pressure level, characterized in that a first booster and a second booster are used for recompressing the third partial stream, the first booster being driven by and mechanically coupled to an expansion machine that is used for the expansion of the third partial stream, the second booster being driven by and mechanically coupled to an expansion machine that is used for the expansion of the second partial stream, and the first booster and the second booster being mechanically uncoupled from one another and from the compressor.

The invention also provides for a plant for the liquefaction of air, with means which are designed for providing a stream of pressurized air at a first pressure level and compressing it by means of a compressor to a second pressure level, forming a first partial stream, a second partial stream and a third partial stream from air of the stream of pressurized air after the compression to the second pressure level, cooling down air of the first partial stream using cold that is produced by means of an expansion of air of the second partial stream and the third partial stream and at least partially liquefying it, and using both feed air that is compressed to the first pressure level and air of the second partial stream and the third partial stream that is provided at the first pressure level for providing the stream of pressurized air at the first pressure level, means being provided that are designed to successively cool down the air of the second partial stream to a first temperature level, allow it to expand from the second pressure level to the first pressure level and heat it with respect to the first partial stream, and are designed furthermore to successively cool down the air of the third partial stream to a second temperature level below the first temperature level, allow it to expand to a third pressure level below the first pressure level, heat it with respect to the first partial stream and recompress it to the first pressure level characterized in that a first booster and a second booster are provided for recompressing the third partial stream, the first booster being able to be driven by and being mechanically coupled to an expansion machine that is used for the expansion of the third partial stream, the second booster being able to be driven by and being mechanically coupled to an expansion machine that is used for the expansion of the second partial stream, and the first booster and the second booster being mechanically uncoupled from one another and from the compressor.

Preferred refinements are respectively also the subject of the dependent patent claims and of the description that follows.

Before the advantages that can be achieved in the context of the present invention are explained, the technical principles on which it is based and some of the terms used in this application are explained in more detail.

An “expansion machine”, which may be coupled to further expansion machines or energy converters such as oil brakes, generators or compressor stages, by way of a common shaft, is designed for the expansion of a supercritical, gaseous or at least partially liquid stream. In particular, for use in the present invention, expansion machines may be designed as turbo expanders. If one or more expansion machines designed as turbo expanders is/are coupled to one or more compressor stages (see below), for example in the form of radial compressor stages, and possibly are additionally mechanically braked, so that the compressor stage(s) is/are operated without energy that is externally supplied, for example by means of an electric motor, the term “booster turbine” is also generally used for this arrangement. The compressor stage(s) of a corresponding booster turbine is or are also referred to as a “booster” or “boosters”. Such a booster turbine compresses at least one stream by the expansion of at least one other stream, but without energy that is externally supplied, for example by means of an electric motor.

On the other hand, a “compressor” is understood here as meaning an externally, typically electrically, driven device, which is designed for compressing at least one gaseous stream from at least one input pressure, at which it is fed to the compressor, to at least one final pressure, at which it is removed from the compressor. The entire compressor in this case forms a structural unit, which however may comprise a number of individual compressor units or “compressor stages” in the form of known piston, screw and/or bucket-wheel or turbine arrangements (that is to say radial or axial compressor stages). In particular, these compressor stages are driven by means of a common drive, for example by way of a common shaft or a common electric motor. A number of compressor stages, e.g. compressor stages in an air liquefaction plant used according to the invention, can consequently together form one or more compressors.

Rotating units, e.g. expansion machines or expansion turbines, compressors or compressor stages, booster turbines or boosters, rotors of electric motors and the like can be mechanically coupled to one another, the term “mechanical coupling” as used in this application being understood to mean that by way of mechanical elements such as gear wheels, belts, gearing and the like, a fixed or mechanically controllable speed relationship is realizable between such rotating units. A mechanical coupling may be realized in general by two or more elements, which are respectively in engagement with one another, for example in locking engagement or frictional engagement, for example gear wheels or driving pulleys with belts, or a rotationally conjoint connection may be established. A rotationally conjoint connection may be brought about in particular by way of a common shaft, on which the rotating units are respectively fastened to rotate therewith. The rotational speed of the rotating units is in this case the same. On the other hand, corresponding units are “mechanically uncoupled” if there is no fixed or mechanically adjustable relationship in terms of rotational speed between corresponding elements. It goes without saying that certain relationships in terms of rotational speed can also be predetermined, for example between multiple electric motors, in particular by suitable electrical activation, or between multiple turbines, in particular by the choice of suitable input pressures and final pressures. However, these are not brought about by two or more elements that are respectively in engagement with one another, for example in locking engagement or frictional engagement, or by a rotationally conjoint connection.

In the context of the present invention, a “heat exchanger” is formed in particular by using one or more counterflow heat exchanger units, for example one or more plate heat exchanger units. By contrast with regenerators for example, here the cooling does not take place by dissipating heat or taking up heat from a solid medium, but indirectly to and from a counterflowing heat or cold transfer medium. All known heat exchanger units, for example plate heat exchangers, tubular heat exchangers and the like, are suitable for use in the present invention. A heat exchanger therefore serves for the indirect transfer of heat between at least two streams made to flow counter to one another, for example a warm stream of compressed air and one or more cold streams or a cryogenic air liquefaction product and one or more warm streams. A heat exchanger may be formed by a single or multiple portions that are connected in parallel and/or in series, for example one or more plate heat exchanger blocks.

For characterizing pressures and temperatures, the present invention uses the terms “pressure level” and temperature level”, with the intention of indicating that pressures and temperatures in a corresponding plant do not have to be used in the form of exact pressure and temperature values in order to realize the inventive concept. However, such pressures and temperatures are typically within certain ranges, which lie for example at ±1%, 5%, 10%, 20% or even 50% around a mean value. Corresponding pressure levels and temperature levels may in this case lie in disjunct ranges or ranges that overlap one another. In particular, for example, pressure levels include unavoidable pressure losses or likely pressure losses, for example as a result of cooling effects or line losses. The same applies correspondingly to temperature levels. The pressure levels indicated here in bar are absolute pressures.

Advantages of the Invention

As explained above, on account of the large amounts of air to be liquefied, air liquefaction for purposes of the storage and recovery of electrical energy may require air liquefaction devices and/or air liquefaction processes that are specially adapted for this. As also explained below with reference to FIG. 1, conventional air liquefaction devices are constructed on the basis of two compressors and two booster turbines:

In what is known as a feed compressor, the entire amount of air to be liquefied, also referred to as feed air, is compressed to about 6 bar. A compressor known as a circulation compressor that is connected downstream of the feed compressor compresses the feed air together with an amount of air returned downstream further from the stated about 6 bar to about 30 to 40 bar. Part of the feed air compressed to the pressure of about 30 to 40 bar is cooled down to differing low temperatures in the form of two partial streams in a heat exchanger. The partial streams are allowed to expand again to the pressure of about 6 bar in one of the expansion machines of the booster turbines in each case, part of the amount of air that has cooled down to the lower temperature being liquefied. The non-liquefied component of the two expanded partial streams is heated in the heat exchanger and, at the pressure of about 6 bar, is returned to the inlet of the circulation compressor. The inlet temperature into one of the two expansion machines lies a temperature between 230 K and ambient temperature and the inlet temperature into the other expansion machine lies at about 140 to 180 K.

A further component of the feed air compressed to the pressure of about 30 to 40 bar is compressed further to about 60 to 80 bar by means of the boosters driven by the said expansion machines. The correspondingly highly compressed stream of air is likewise cooled down in the heat exchanger and allowed to expand at a suitable temperature by means of a throttle. The air of this stream of air is also thereby liquefied, at least partially. The air that is liquefied altogether is therefore formed by air of the stream of air compressed to the pressure of about 60 to 80 bar and air that is fed to the expansion machine at the inlet temperature of about 140 to 180 K. The amount of air to be liquefied is in this case at least partially cooled by the air allowed to expand in the expansion machines.

A disadvantage of the process just explained is that it is designed for the final pressure of the feed compressor, and therefore allows little freedom. Both the inlet pressure of the circulation compressor and the pressure at the outlet of the two expansion machines are in this way predetermined, to be specific to the final pressure of the feed compressor or the inlet pressure of the circulation compressor.

However, feed compressors with a relatively high outlet pressure of about 12 to 20 bar would be advantageous, especially for the storage of energy or for air liquefaction devices that can be used for this purpose. Correspondingly, in the conventional process also the pressure, but also the minimum temperature, at the outlet of the expansion machines would be fixed, in particular of the expansion machine operated at lower temperatures. The reason for this is that the liquid component at the outlet from a corresponding expansion machine typically must not be any more than 6 to 8 percent. This temperature would be higher in the case of the process explained above, in which the outlet pressure of the feed compressor or the inlet pressure of the circulation compressor is about 6 bar. The amount of air primarily to be liquefied, i.e. air compressed to the still higher pressure of about 60 to 80 bar, would be pre-cooled with the cold stream from the explained expansion machine, but on account of the circumstances explained not as much as in a conventional process. In comparison with the conventional process, the temperature of this stream at the coldest point would be much higher, that is at about 111 to 120 K instead of 101 K. After the subsequent expansion in the explained throttle, much more steam would therefore be produced, and comparatively less air liquefaction product, which suggests greater losses.

The invention achieves the object of improving a corresponding process by the outlet pressure of the explained expansion machine that is operated at a lower temperature being reduced, so that this pressure becomes lower than the final pressure of the feed compressor or the inlet pressure of the circulation compressor. The amount of air allowed to expand in this expansion machine is in this case not passed directly to the inlet of the circulation compressor (or to it via the heat exchanger), but is initially recompressed to the final pressure of the feed compressor or the inlet pressure of the circulation compressor in two boosters.

In this way it is achieved that an expansion to lower pressures is possible even in cases of relatively high final pressures of a feed compressor or inlet pressures of a circulation compressor, so that effective cooling for the liquefaction of air is made possible. In the context of the invention, losses in downstream devices are also reduced in this way.

Against this background, the present invention proceeds from a process for the liquefaction of air in which a stream of pressurized air is provided at a first pressure level and is compressed by means of a compressor, namely by means of the circulation compressor explained, to a second pressure level, a first partial stream, a second partial stream and a third partial stream are formed from air of the stream of pressurized air after the compression to the second pressure level, air of the first partial stream is cooled down using cold that is produced by means of an expansion of air of the second partial stream and the third partial stream and is at least partially liquefied, and in which feed air that is compressed to the first pressure level and air of the second partial stream and the third partial stream that is provided at the first pressure level are used for providing the stream of pressurized air at the first pressure level.

The present invention provides here that the air of the second partial stream is successively cooled down to a first temperature level, allowed to expand from the second pressure level to the first pressure level and heated with respect to the first partial stream, and that the air of the third partial stream is successively cooled down to a second temperature level below the first temperature level, allowed to expand to a third pressure level below the first pressure level, heated with respect to the first partial stream and recompressed to the first pressure level. The expansion to the third pressure level below the first pressure level has the effect of achieving the advantages explained above, to be specific improved production of cold, which is used in the area of production of cold in conventional plants with a corresponding feed-compressor final pressure of about 6 bar or such a circulation-compressor inlet pressure. At the same time, the advantages of a higher-compressing feed compressor can be exploited.

According to the invention, a first booster and a second booster are used here for recompressing the third partial stream, the first booster being driven by and mechanically coupled to an expansion machine that is used for the expansion of the air of the second partial stream and the second booster being driven by and mechanically coupled to an expansion machine that is used for the expansion of the air of the third partial stream. This makes it possible to use power that is released during the expansion of the partial streams effectively for the recompression of the air of the third partial stream. No additional, externally driven compressors are required. According to the invention, the first booster and the second booster are mechanically uncoupled from one another and from the (circulation) compressor in the sense explained above. Consequently, the two expansion machines mentioned are also mechanically uncoupled with respect to one another and from the compressor. In this way, the compressor, the first booster with the expansion machine that is used for the expansion of the air of the second partial stream and the second booster with the expansion machine that is used for the expansion of the air of the third partial stream can be respectively operated independently of one another (within operational limits). In particular, as a result, the cold provided by expansion of the second and third partial streams and also the outlet pressures of the boosters can be set individually and independently of the compressor. In particular, the third pressure level can be set independently.

The explained mechanical relationships of the boosters and expansion machines with respect to one another and with the compressor have the effect that the recompression of the air of the partial stream to the first pressure level and the compression of the stream of pressurized air to the first pressure level take place independently of one another, i.e. in different apparatuses (the first booster and the second booster on the one hand and in the compressor on the other hand).

According to a particularly preferred embodiment of the present invention, air of the third partial stream, which remains gaseous at the third pressure level and a third temperature level produced by the expansion to the third pressure level, is used as the air of the third partial stream, which after the cooling down to the second temperature level and the expansion to the third pressure level is heated with respect to the first partial stream and is recompressed to the first pressure level. In other words, according to this embodiment of the present invention, after the expansion of the third partial stream to the third pressure level, a liquid phase is separated and only the air of the third partial stream is passed through a corresponding heat exchanger and used for the cooling down of the first partial stream, which here remains gaseous.

According to this particularly preferred embodiment of the invention, it may be provided that air of the third partial stream, which has been cooled down to the second temperature level and allowed to expand to the third pressure level and is in liquid form at the third temperature level and the third pressure level, is combined with liquefied air of the first partial stream. Such air has already been liquefied, so that it can advantageously be used for obtaining the air liquefaction product in a way corresponding to the process according to the invention.

Advantageously, after cooling down, the air of the first partial stream is allowed to expand to a fourth pressure level below the third pressure level. Advantageously used for this purpose is a throttling device, for example a throttle valve or a generator turbine, in which further cold is generated by Joule-Thomson expansion, and consequently better liquefaction of the air is brought about.

Advantageously, before the air of the first partial stream is allowed to expand to the fourth pressure level, this air is first allowed to expand to the third pressure level explained above. In this way, it is made possible for components of the first partial stream to be combined with for example liquefied air of the third partial stream that is at the third pressure level.

It is particularly advantageous in this case to combine the air of the third partial stream, which after cooling down to the second temperature level and expansion to the third pressure level is heated with respect to the first partial stream and recompressed to the first pressure level, after expansion to the third pressure level with air of the first partial stream that has been allowed to expand to the third pressure level and remains gaseous. Corresponding air can therefore be used effectively and be returned to the inlet of a circulation compressor.

Advantageously, in the context of the present invention, the first pressure level lies at 5 to 25 bar, in particular at 10 to 20 bar, and/or the second pressure level lies at 50 to 100 bar, in particular at 60 to 80 bar. For example, about 17 bar may be used as the first pressure level and about 70 bar may be used as the second pressure level. As explained above, the process according to the invention is therefore particularly suitable for processes for the liquefaction of air that are used in processes for the storage and recovery of energy, where comparatively large amounts of air have to be liquefied. In the same way, the process according to the invention is however also suitable for other scenarios of use in which corresponding requirements exist. In particular, use of the process according to the invention allows the use of feed compressors, which provide correspondingly high first pressure levels.

Advantageously, the third pressure level lies at least 1, 5 or 10 bar and up to 20 bar below the second pressure level and/or the fourth pressure level lies 1, 5 or 10 bar and up to 20 bar below the third pressure level, the fourth pressure lying in particular at atmospheric pressure. An example of the third pressure level is about 6.5 bar. A corresponding expansion to such a low third pressure level that corresponds to the feed-compressor final pressure and/or circulation-compressor inlet pressure in the conventional processes explained above makes particularly effective cooling down possible.

Advantageously, the first temperature level lies at 230 to 330 K and/or the second temperature level lies at 140 to 180 K. Corresponding temperature levels correspond to those conventional processes as explained above, so that empirical values used thereby can continue to be used.

The invention also relates to a process for the storage and recovery of electrical energy, which comprises a first operating mode, in which air is liquefied by means of electrical energy, and a second operating mode, in which electrical energy is obtained by using the air liquefied in the first operating mode. The first operating mode is in this case the operating mode explained at the beginning at times when electric power is inexpensive or times when there is a surplus of electric power, that is to say in an energy storage period, the second operating mode is the operating mode that is used at times of peak load, that is to say in an energy recovery period. A corresponding process for the storage and recovery of electrical energy is distinguished according to the invention by the fact that a process as explained above is used in the first operating mode. With respect to this process, reference is therefore expressly made to the features and advantages that have been explained above.

The invention also relates to a plant for the liquefaction of air. This has means which are designed for providing a stream of pressurized air at a first pressure level and compressing it by means of a compressor, of the circulation compressor which has been mentioned multiple times, to a second pressure level, forming a first partial stream, a second partial stream and a third partial stream from air of the stream of pressurized air after the compression to the second pressure level, cooling down air of the first partial stream using cold that is produced by means of an expansion of air of the second partial stream and the third partial stream and at least partially liquefying it, and using feed air that is compressed to the first pressure level and air of the second partial stream and the third partial stream that is provided at the first pressure level for providing the stream of pressurized air at the first pressure level.

Means are provided that are designed to successively cool down the air of the second partial stream to a first temperature level, allow it to expand from the second pressure level to the first pressure level and heat it with respect to the first partial stream, and are designed furthermore to successively cool down the air of the third partial stream to a second temperature level below the first temperature level, allow it to expand to a third pressure level below the first pressure level, heat it with respect to the first partial stream and recompress it to the first pressure level.

According to the invention, a first booster and a second booster are provided for recompressing the third partial stream, the first booster being able to be driven by and being mechanically coupled to an expansion machine that is used for the expansion of the third partial stream, the second booster being able to be driven by and being mechanically coupled to an expansion machine that is used for the expansion of the second partial stream, and the first booster and the second booster being mechanically uncoupled from one another and from the compressor.

For features and advantages of a corresponding plant, reference is likewise made to the features and advantages explained above. In particular, if a corresponding plant is designed for carrying out a process as explained above, it may therefore also be a plant for the storage and recovery of electrical energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to the accompanying drawings, in which embodiments of the invention are illustrated in comparison with the prior art.

FIG. 1 illustrates a plant not according to the invention for the liquefaction of air in the form of a schematic process flow diagram.

FIG. 2 illustrates a plant for the liquefaction of air according to one embodiment of the invention in the form of a schematic process flow diagram.

FIG. 3 illustrates a plant for the liquefaction of air according to one embodiment of the invention in the form of a schematic process flow diagram.

FIG. 4 illustrates a plant for the liquefaction of air according to one embodiment of the invention in the form of a schematic process flow diagram.

FIG. 5 illustrates a plant for the liquefaction of air according to one embodiment of the invention in the form of a schematic process flow diagram.

In the figures, elements, streams and devices that correspond to one another are indicated by identical designations and, for the sake of overall clarity, are not explained more than once. Fluid streams are respectively indicated by upper-case or lower-case letters; fluid streams that are predominantly or exclusively gaseous are also illustrated by arrows that are not filled in (white), whereas fluid streams that are predominantly or exclusively liquid are illustrated by arrows that are filled in (black).

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a plant not according to the invention for the liquefaction of air, which is designated overall by 500. For the sake of better distinguishability, the fluid streams are indicated here by upper-case letters.

The plant 500 is fed feed air A at ambient pressure, which after being combined with a further stream of air X, is compressed in a compressor 12, known as the feed compressor. Connected downstream of the feed compressor 12 there may be an aftercooler, which is not separately designated. A correspondingly obtained stream of pressurized air, now designated by B, is fed to a second compressor 11, known as the circulation compressor, likewise downstream of which there is connected an aftercooler, which is not separately designated. The circulation compressor 11 is likewise fed a stream Y, which is formed as explained below. The streams B and Y are further compressed in the circulation compressor 11. Downstream of the circulation compressor 11, a stream obtained by the compression, which is now designated by C, is divided into a first partial stream D and a second partial stream E.

The first partial stream D is fed to a heat exchanger 13 on the warm side and removed from it at an intermediate temperature level. The first partial stream is subsequently allowed to expand to the pressure level provided by the feed compressor 12 in a first expansion machine 14 and fed to the heat exchanger 13 at an intermediate temperature level.

The second partial stream E is first compressed to a higher pressure level in a (“first”) booster 15 and subsequently in a (“second”) booster 16, downstream of which there may be connected an aftercooler, which is not separately designated. The correspondingly compressed second partial stream E is likewise fed to the heat exchanger 13 on the warm side and partially removed from it at an intermediate temperature level in the form of the stream F. The stream F is allowed to expand in an expansion machine 17 and subsequently transferred into a separator vessel 18. The expansion in the expansion machine 17 likewise takes place to the pressure level provided by the feed compressor 12.

The separator vessel 18 is likewise fed a second component of the second partial stream E, which is designated here by G and is taken almost right up to the cold end through the heat exchanger 13. Liquid air separated in the sump of the separator vessel 18 is drawn off in the form of the stream H, supercooled in part of the heat exchanger 13 and subsequently partially transferred, for example into a tank. Used inter alia for the supercooling is cold that can be obtained from an expansion of part of the stream H to form the mentioned stream X. The latter is subsequently passed through the heat exchanger 13 from the cold end to the warm end. The air remaining gaseous in the separator vessel 18 is drawn off in the form of the stream I, heated in the heat exchanger 13 and subsequently combined in the form of the stream Y with the stream of pressurized air B explained above.

As mentioned above, the air liquefaction plant 100 illustrated in FIG. 1 has disadvantages. In the feed compressor 12, the feed air of the stream A is compressed to approximately 6 bar. The circulation compressor 11 compresses the feed air of the stream A together with a returned amount of air of the stream Y further from the stated about 6 bar to about 30 to 40 bar. In the expansion machine 14, the air of the stream D compressed to the pressure of about 30 to 40 bar is then allowed to expand once again to the pressure of about 6 bar and at this pressure level is returned as part of the stream Y to the inlet of the circulation compressor 11. In the expansion machine 17, the air of the stream F, first compressed to the pressure of about 30 to 40 bar and subsequently compressed further in the boosters 15 and 16 to about 60 to 80 bar, is likewise allowed to expand to the pressure of about 6 bar. A component of this that in this case remains gaseous in the separator vessel 18 is likewise returned as part of the stream Y to the inlet of the circulation compressor 11.

The expansion machines 14 and 17 are placed in such a way that the inlet temperature into the expansion machine 14 lies at a temperature level of between 230 K and ambient temperature and the inlet temperature of the expansion machine 17 lies at about 140 to 180 K. This is achieved by the streams D and F being passed through the heat exchanger 13 and being removed from it at the stated temperatures. The boosters 15 and 16 driven by the said expansion machines 14 and 17 compress the stream E to the stated pressure level of about 60 to 80 bar. Part of the stream E compressed to the pressure level of about 60 to 80 bar is allowed to expand into the separator vessel 18 by way of a throttle in the form of the stream G. The air of this stream is thereby liquefied, at least partially. The liquefied air of the stream H is formed by the liquefied air of the stream G and by liquefied air of the stream F allowed to expand in the expansion machine 17. The amount of air to be liquefied is in this case respectively cooled by the air allowed to expand in the expansion machines 14 and 17.

A disadvantage of this is that a corresponding process is to be designed for the final pressure of the feed compressor 12, and therefore allows little freedom. Both the inlet pressure of the circulation compressor 11 and the pressure at the outlet of the two expansion machines 14 and 17 are in this way predetermined, to be specific to the final pressure of the feed compressor 12 and the inlet pressure of the circulation compressor 11. Feed compressors 12 with a higher final pressure of about 12 to 20 bar would be advantageous. Correspondingly, however, in the process implemented in the air liquefaction plant 500 also the pressure and the minimum temperature at the outlet of the expansion machines 14 and 15 would be correspondingly fixed, in particular of the expansion machine 17 operated at lower temperatures. The liquid component at the outlet from such an expansion machine 17 may typically be no more than 6 to 8 percent. However, the temperature to be maintained for this purpose then lies higher than in the case of a process in which the outlet pressure of the feed compressor 12 and/or the inlet pressure of the circulation compressor 11 is about 6 bar. The amount of air to be liquefied, i.e. air of the stream E compressed to the third pressure level of about 60 to 80 bar, would be pre-cooled with the cold stream F from the expansion machine 17, but on account of the circumstances explained not as much as in a conventional process, in which the outlet pressure of the feed compressor 12 and/or the inlet pressure of the circulation compressor 11 is about 6 bar. In comparison with a conventional process, the final temperature would be much higher, that is at about 111 to 120 K instead of 101 K. After the subsequent expansion in the explained throttle, much more steam would therefore be produced, and comparatively less air liquefaction product, which suggests greater losses.

In FIG. 2, a plant for the liquefaction of air according to one embodiment of the invention is illustrated in the form of a schematic process flow diagram and is designated overall by 100.

The plant 100 is fed feed air a at a first pressure level. Here, too, “feed air” is understood as meaning air that is provided externally and is for example freed of water and/or carbon dioxide by means of suitable cleaning devices, and this air is compressed by means of a feed compressor not represented in FIG. 2 to a pressure level (“first pressure lever”) that lies much higher here however than in the plant 500, for example at the mentioned about 12 to 20 bar.

By combining with a further stream of air g at the first pressure level (see below with respect to stream g), a stream of pressurized air b is formed and compressed further to a “second” pressure level by means of a compressor, the circulation compressor 11, which has been mentioned multiple times but which can optionally be configured differently from the circulation compressor 11 explained with reference to the plant 500 according to FIG. 1. Here, too, connected downstream of the circulation compressor 11 there may be an aftercooler, which is not separately designated. Finally, three partial streams c, e and f are formed from the stream of pressurized air b compressed to the second pressure level. The partial stream c is in this case fed to a heat exchanger 2 on the warm side and removed from it on the cold side, allowed to expand and thereby cooled down and at least partially liquefied. For the cooling down and consequently for the at least partial liquefaction of the partial stream c, cold that is produced by means of an expansion of air of the second partial stream e and the third partial stream f, as explained below, is used.

The air of the second partial stream e and the third partial stream f is first fed jointly to the heat exchanger 2 on the warm side. The air of the second partial stream e is removed from the heat exchanger 2 at a first temperature level, the air of the third partial stream f is removed at a second temperature level, the second temperature level lying below the first temperature level. The air of the second partial stream e is allowed to expand again to the first pressure level in a first expansion machine 3, thereby cooled down further, fed to the heat exchanger 2 at an intermediate temperature, removed from the heat exchanger 2 on the warm side and correspondingly heated, and subsequently combined with the air of the third partial stream f, which is treated in the way explained below.

The third partial stream f is fed to a second expansion machine 4, is allowed to expand in it and is thereby likewise cooled down. While the air of the second partial stream e is allowed to expand in the first expansion machine 3 to the first pressure level, at which the feed air a is also provided and the stream of pressurized air b is present, the expansion of the third partial stream f in the second expansion machine 4 is however to a “third” pressure level, below the first pressure level. The third partial stream f is fed to the heat exchanger 2 on the cold side and removed on the warm side. Subsequently, the third partial stream f is recompressed to the first pressure level by means of two boosters 5 and 6, downstream of which there may be connected aftercoolers, which are not separately designated and which are respectively mechanically coupled to the second expansion machine 4 (booster 5) and the first expansion machine 3 (booster 6). The boosters 5, 6 and the expansion machines 3, 4 are mechanically uncoupled with respect to one another and from the circulation compressor 11. As already explained, this is followed by the air of the third partial stream f being combined with the air of the second partial stream e, in the example to form the already explained partial stream g.

The cold produced by means of the expansion in the expansion machines 3 and 4 is introduced into the heat exchanger 2 and serves here for the cooling down and at least partial liquefaction of the first partial stream c. The at least partially liquefied air of the first partial stream c is fed to an expansion device 7, which may for example comprise a generator turbine and one or more expansion valves, and is allowed to expand in it. The air of the first partial stream c that has correspondingly been allowed to expand is subsequently transferred into a separator vessel 8, in the sump of which liquefied air is separated and can be drawn off as stream of liquefied air h and stored. From the top of the separator vessel 8, gaseous air of the first stream of pressurized air c is drawn off in the form of the stream i, fed to the heat exchanger 2 on the cold side and removed from it on a warm side.

As explained with reference to FIG. 3, which illustrates a variant of the plant 100 according to FIG. 2 that is designated overall by 200, the entire air of the third partial stream f does not have to be recompressed from the third pressure level, which lies below the first pressure level, to the first pressure level. In the second expansion machine 4, the air of the third partial stream f may also be allowed to expand in such a way that the air of the third partial stream f is partially liquefied. The entire air of the third partial stream f may therefore first be fed into a separator vessel 9, in the sump of which liquid air is separated and can be drawn off as stream of liquid air k and combined with the air of the first partial stream c. Remaining air of the third partial stream f, which remains gaseous in the separator vessel 9, can, as explained above with respect to the entire third partial stream f, be fed as stream I to the heat exchanger 2 on the cold side, removed on the warm side, and subsequently recompressed to the first pressure level. In the plants 100 and 200 represented in FIGS. 2 and 3, the separator vessel 8 is operated below the third pressure level at any desired “fourth” pressure level, so that in the plant 200 illustrated in FIG. 3 the stream of liquid air k has to be allowed to expand from the third pressure level to the fourth pressure level.

A further variant of a plant according to the invention is represented in FIG. 4 and designated overall by 300. Here, too, a separator vessel used at the fourth pressure level explained above is used, and is therefore likewise designated here by 8. A further separator vessel, which is operated at the third pressure level, i.e. at the pressure level at which the separator vessel 9 according to plant 200 or FIG. 3 is operated, is likewise designated here by 9. However, the air of the first partial stream c, which is allowed to expand in the expansion device 7, is only allowed to expand here to the third pressure level and at this level is transferred into the separator vessel 9. Liquefied air from the sump of the separator vessel 9 is allowed to expand to the fourth pressure level and transferred into the separator vessel 8 in the form of the stream m. Air remaining gaseous is drawn off from the top of the separator vessel 9 at the third pressure level as stream n and is combined with the air of the third partial stream f to form a collective stream o. The collective stream o is further treated as explained above with reference to the stream l in FIG. 3 or plant 200, i.e. is heated in the heat exchanger 2 and subsequently recompressed to the first pressure level.

In FIG. 5, a further variant of a plant according to the invention is represented and is denoted overall by 400. This also has two separator vessels, which on account of the pressure levels respectively used are likewise designated here by 8 and 9. In the plant 400 according to FIG. 5, the air of the third partial stream f at the third pressure level is combined with the air of the first partial stream c, which has likewise been allowed to expand to the third pressure level in the expansion device 7, to form a collective stream p and is fed into the separator vessel 9 at the third pressure level. Liquid air occurring in the separator vessel 9 is allowed to expand to the fourth pressure level and transferred into the separator vessel 8 in the form of the stream r. The liquid air of the stream of liquid r is in this case air that has been formed from air of the first partial stream c and from air of the third partial stream f. Air remaining gaseous in the separator vessel 9 is drawn off in the form of the stream s. The air of the stream s is consequently likewise air of the first partial stream c and air of the third partial stream f. This is treated as explained above with reference to the streams I with respect to plant 200 or FIG. 3 and stream o with respect to plant 300 or FIG. 4. 

What I claim is:
 1. A process for the liquefaction of air in which a stream of pressurized air is provided at a first pressure level and is compressed using a compressor to a second pressure level, a first partial stream, a second partial stream and a third partial stream are formed from air of the stream of pressurized air after the compression to the second pressure level, air of the first partial stream is cooled down using cold that is produced by means of an expansion of air of the second partial stream and the third partial stream and is at least partially liquefied, and both feed air that is compressed to the first pressure level and air of the second partial stream and the third partial stream that is provided at the first pressure level are used for providing the stream of pressurized air at the first pressure level, the air of the second partial stream being successively cooled down to a first temperature level, allowed to expand from the second pressure level to the first pressure level and heated with respect to the first partial stream, and the air of the third partial stream being successively cooled down to a second temperature level below the first temperature level, allowed to expand to a third pressure level below the first pressure level, heated with respect to the first partial stream and recompressed to the first pressure level, characterized in that a first booster and a second booster are used for recompressing the third partial stream, the first booster being driven by and mechanically coupled to an expansion machine that is used for the expansion of the third partial stream, the second booster being driven by and mechanically coupled to an expansion machine that is used for the expansion of the second partial stream, and the first booster and the second booster being mechanically uncoupled from one another and from the compressor.
 2. The process according to claim 1, in which a common shaft is used in each case for the mechanical coupling of the first booster to the expansion machine that is used for the expansion of the third partial stream and the second booster to the expansion machine that is used for the expansion of the second partial stream.
 3. The process according to claim 1, in which air of the third partial stream, which remains gaseous at the third pressure level and a third temperature level produced by the expansion to the third pressure level, is used as the air of the third partial stream, which after the cooling down to the second temperature level and the expansion to the third pressure level is heated with respect to the first partial stream and is recompressed to the first pressure level.
 4. The process according to claim 3, in which air of the third partial stream, which has been cooled down to the second temperature level and allowed to expand to the third pressure level and is in liquid form at the third temperature level and the third pressure level, is combined with liquefied air of the first partial stream.
 5. The process according to claim 1, in which, after cooling down and at least partially liquefying, the air of the first partial stream is allowed to expand to a fourth pressure level below the third pressure level.
 6. The process according to claim 5, in which, before being allowed to expand to the fourth pressure level, the air of the first partial stream is allowed to expand to the third pressure level.
 7. The process according to claim 6, in which the air of the third partial stream, which after cooling down to the second temperature level and expansion to the third pressure level is heated with respect to the first partial stream and recompressed to the first pressure level, after expansion to the third pressure level is combined with air of the first partial stream that has been allowed to expand to the third pressure level and remains gaseous.
 8. The process according to claim 1, in which the first pressure level lies at 5 to 25 bar and/or in which the second pressure level lies at 50 to 100 bar.
 9. The process according to claim 1, in which the third pressure level lies at least 1 bar below the second pressure level and/or the fourth pressure level lies at least 1 bar below the third pressure level.
 10. The process according to claim 1, in which the first temperature level lies at 230 to 330 K and/or in which the second temperature level lies at 140 to 180 K.
 11. The process according to claim 8, in which the first pressure level lies at 10 to 20 bar and/or the second pressure level lies at 60 to 80 bar.
 12. The process according to claim 9, in which the third pressure level lies at least 5 bar below the second pressure level and/or the fourth pressure level lies at least 5 bar below the third pressure level.
 13. The process according to claim 9, in which the third pressure level lies at least 10 bar below the second pressure level and/or the fourth pressure level lies at least 10 bar below the third pressure level.
 14. The process according to claim 9, in which the third pressure level is at most 20 bar below the second pressure level and/or the fourth pressure level lies at most 20 bar below the third pressure level.
 15. The process according to claim 9, in which the fourth pressure level lies at atmospheric pressure.
 16. The process according to claim 1, in which a first operating mode, in which air is liquefied by means of electrical energy, and a second operating mode, in which electrical energy is obtained by using the air liquefied in the first operating mode, characterized in that the process is carried out in the first operating mode.
 17. A plant for the liquefaction of air, with means which are designed for providing a stream of pressurized air at a first pressure level and compressing it by means of a compressor to a second pressure level, forming a first partial stream, a second partial stream and a third partial stream from air of the stream of pressurized air after the compression to the second pressure level, cooling down air of the first partial stream using cold that is produced by means of an expansion of air of the second partial stream and the third partial stream and at least partially liquefying it, and using both feed air that is compressed to the first pressure level and air of the second partial stream and the third partial stream that is provided at the first pressure level for providing the stream of pressurized air at the first pressure level, means being provided that are designed to successively cool down the air of the second partial stream to a first temperature level, allow it to expand from the second pressure level to the first pressure level and heat it with respect to the first partial stream, and are designed furthermore to successively cool down the air of the third partial stream to a second temperature level below the first temperature level, allow it to expand to a third pressure level below the first pressure level, heat it with respect to the first partial stream and recompress it to the first pressure level characterized in that a first booster and a second booster are provided for recompressing the third partial stream, the first booster being able to be driven by and being mechanically coupled to an expansion machine that is used for the expansion of the third partial stream, the second booster being able to be driven by and being mechanically coupled to an expansion machine that is used for the expansion of the second partial stream, and the first booster and the second booster being mechanically uncoupled from one another and from the compressor.
 18. The plant according to claim 17, in which the first booster is coupled to the expansion machine that is used for the expansion of the third partial stream and the second booster is coupled to the expansion machine that is used for the expansion of the second partial stream by means of a common shaft respectively. 